Lithography in semiconductor processing relates generally to the process of transferring patterns which correspond to desired circuit components onto one or more thin films which overlie a substrate. One important step within the field of lithography involves optical tools and methods for transferring the patterns to the films which overlie the semiconductor wafer. Patterns are transferred to a film by imaging various circuit patterns onto a photoresist layer which overlies the film on the wafer. This imaging process is often referred to as "exposing" the photoresist layer. The benefit of the exposure process and subsequent processing allows for the generation of the desired patterns onto the film on the semiconductor wafer, as illustrated in prior art FIGS. 1a-1f.
Prior art FIG. 1a illustrates a photoresist layer 10 deposited by, for example, spin-coating, on a thin film 11 such as silicon dioxide (SiO.sub.2) which overlies a substrate 12 such as silicon. The photoresist layer 10 is then selectively exposed to radiation 13 (e.g., ultraviolet (UV) light) via a photomask 14 (hereinafter referred to as a "mask") to generate one or more exposed regions 16 in the photoresist layer 10, as illustrated in prior art FIG. 1b. Depending on the type of photoresist material utilized for the photoresist layer 10, the exposed regions 16 become soluble or insoluble in a specific solvent which is subsequently applied across the wafer (this solvent is often referred to as a developer).
When the exposed regions 16 are made soluble, a positive image of the mask 14 is produced in the photoresist layer 10, as illustrated in prior art FIG. 1c, and the photoresist material is therefore referred to as a "positive photoresist". The exposed underlying areas 18 in the film 11 may then be subjected to further processing (e.g., etching) to thereby transfer the desired pattern from the mask 14 to the film 11, as illustrated in prior art FIG. 1d (wherein the photoresist layer 10 has been removed). Conversely, when the exposed regions 16 are mode insoluble, a negative image of the mask 14 is produced in the photoresist 10 layer, as illustrated in prior art FIG. 1e, and the photoresist material is therefore referred to as a "negative photoresist." In a similar manner, the exposed underlying areas 20 in the film 11 may then be subjected to further processing (e.g., etching) to thereby transfer the desired pattern from the mask 14 to the film 11, as illustrated in prior art FIG. 1f.
The transfer of patterns to the photoresist layer 10 as discussed above involves the use of optical aligners. Optical aligners are machines which contain a variety of subsystems that work together to form the imaging function. Such optical aligners include: (1) an illumination source which provides the optical energy (UV light in the above example) for transforming the photoresist via exposure, (2) an optical subsystem that focuses the circuit patterns onto the photoresist surface and allows for controlled exposure times, and (3) a movable stage that holds the wafer being exposed.
Historically, three primary methods have been used to optically transfer a mask pattern to a photoresist covered film. These methods include: contact printing, proximity printing and projection printing and are illustrated in simplified form in prior art FIGS. 2a-2d, respectively. Contact printing 100, as illustrated in prior art FIG. 2a, was the earliest method used to produce patterns. Contact printing 100 involves a light source 112, an optical system 114, a mask 116 and a photoresist layer 118 overlying a thin film which, in turn, overlies a semiconductor wafer 120. The mask 116, which contains the desired circuit patterns for transfer to the photoresist layer 118, is positioned (aligned) relative to any existing patterns that already exist on the wafer 120. The mask 116 is then clamped down to the photoresist layer 118, thereby making physical contact with the photoresist layer 118, and exposed with ultraviolet (UV) light from the light source 112. This method provides for an excellent image transfer and good resolution (e.g., good minimum linewidth spacing).
Contact printing, however, suffers from the direct contact made between the mask 116 and the photoresist layer 118. The repeated contact made between the mask 116 and the photoresist layer 118 in the process results in defects generated in the mask 116 which are in turn transferred to subsequently processed wafers. To prevent this problem, the masks 116 must be inspected and cleaned regularly which can be disadvantageous in terms of cost and processing time. In addition, small particles may be caught between the mask 116 and the photoresist layer 118 when affixing the two elements, thereby preventing the desired direct contact between the mask 116 and the photoresist layer 118. This particulate contamination results in reduced resolution in the area local to the foreign particle. Consequently, contact printing is not common in VLSI semiconductor manufacturing.
Proximity printing 122, as illustrated in prior art FIG. 2b, involves placing the mask 116 near the wafer 120 (which is covered with the photoresist 118) during exposure, however, the mask 116 and the wafer 120 do not make contact. By introducing a gap 124 between the mask 116 and the wafer 120, the defect problem of contact printing is substantially avoided. Unfortunately, as the gap 124 increases, the resolution of the proximity printing system 122 rapidly deteriorates. For example, a 10 .mu.m gap with a 400 nm exposure (the wavelength of the light source 112) results in a minimum resolution of about 3 .mu.m. In addition, proximity printing 122 requires extremely flat masks 116 and wafers 120 in order to prevent gap variations spatially about the wafer 120. Since many VLSI semiconductor circuits today require features of 0.25 .mu.m or less, proximity printing 122 is not considered adequate for many VLSI semiconductor manufacturing operations.
Projection printing is a generic term that encompasses various pattern transfer techniques. These techniques, for example, include: (a) projection scanning systems, (b) reduction (e.g., 4.times. or 10.times.) step-and-repeat projection systems, and (c) reduction step-and-scan systems. In each system, lens elements or mirrors are used to focus the mask image on the wafer surface (containing the photoresist).
Projection scanning systems (often called scanning projection aligners), use a reflective spherical mirror (reflective optics) to project an image onto the wafer surface, as illustrated, for example, in prior art FIG. 2c. The system 126 includes a primary mirror 128 and a secondary mirror 129 which are arranged with the mask 116 and the wafer 120 to image the mask pattern onto the photoresist layer 118 which overlies the film on the wafer 120 (the photoresist layer 118 and the thin film are not shown in FIG. 2c for simplicity). A narrow arc of radiation is imaged from the mask 116 to the wafer 120 through a slit with light that travels an optical path that reflects the light multiple times. The mask 116 and the wafer 120 are scanned through the arc of radiation by means of a continuous scanning mechanism (not shown). The scanning technique minimizes mirror distortions and aberrations by keeping the imaging illumination in the "sweet spot" of the imaging system 128 and 129.
Reduction step-and-repeat systems 130 (also called reduction steppers) use refractive optics (as opposed to reflective optics in the system 126 of prior art FIG. 2c) to project the mask image onto the photoresist layer 118 which overlies the film on the wafer 120, as illustrated, for example, in prior art FIG. 2d. The reduction stepper 130 includes a mirror 132, a light source 134, a filter 136, a condenser lens system 138, a reticle 140, a reduction lens system 142 and the wafer 120. The mirror 132 behaves as a collecting optics system to direct as much of the light from the light source 134 (e.g., a mercury-vapor lamp) to the wafer 120. The filter 136 is used to limit the light exposure wavelengths to the specified frequencies and bandwidth. The condenser system 138 focuses the radiation through the reticle 140 and to the reduction lens system to thereby focus a "masked" radiation exposure onto a limited portion of the wafer 120, namely onto a single semiconductor die 144.
Since it is complex and expensive to produce a lens capable of projecting a mask pattern of an entire 150 mm or 200 mm wafer, the refractive system 130, as illustrated in prior art FIG. 2d, projects an image only onto a portion of the wafer 120 corresponding to an individual semiconductor die 144. This image is then stepped and repeated across the wafer 120 in order to transfer the pattern to the entire wafer (and thus the name "steppers"). Consequently, the size of the wafer is no longer a consideration for the system optics.
The reduction stepper system 130 thus uses the reticle 140 instead of a mask. Reticles are similar to masks, but differ in that a mask contains a pattern for transfer to the entire wafer in one exposure while a reticle contains a pattern image for a single or several semiconductor die that must be stepped and repeated across the wafer 120 in order to expose the entire wafer substrate. Today, the term "mask" and "reticle" are used interchangeably and will be used interchangeably hereinafter. Current reduction stepper systems such as the system 130 utilize reticles that contain a pattern that is an enlargement of the desired image on the wafer 120. Consequently, the reticle pattern is reduced when projected onto the wafer 120 during exposure (and thus the name "reduction stepper").
Steppers are typically evaluated by measuring the critical dimension of a feature produced by the stepper. A critical dimension is typically defined as the absolute size of a minimum feature, including the feature linewidth and feature spacing. The resulting critical dimension of a feature measured on the wafer is a function of the critical dimension contributions or components provided by the stepper and the reticle. Thus, to characterize a stepper being used as a lithographic printer, it is desirable to separate out the critical dimension components attributable to the reticle from critical dimension components which are attributable to the imaging system. In this manner, variations in the critical dimension of features across a die can be properly attributed either to the reticle or to the imaging system of the stepper. With such distilled information, the reticle or the imaging system (or both) can be properly characterized.
Variations in the critical dimension due to the reticle are sometimes caused by reticle fabrication defects. Such defects include, for example, bubbles, scratches, pits and fractures of the reticle glass substrate. Additional defects may include chrome defects such as particulate inclusions in the film, pinholes or voids in the chrome surface and invisible chemical anomalies which lead to erratic local etching. Still additional defects may be caused by resist defects such as voids which produce pinholes that lead to chrome spots. Furthermore, critical dimension variations are also due to variations in the process conditions during the exposure and development process such as variations in the exposure dose, development rate, and temperature.
Various techniques and inspection tools exist and are used to verify whether a fabricated reticle meets the minimum required quality specifications. Such inspection tools typically utilize the transmission of light using an automated defect detection system. Although these tools can separate out unacceptable reticles which fail to meet the required minimum specifications, these tools do not characterize the variations in the reticle which give rise to variations in the critical dimension of patterned features across the die. Thus, such inspection tools do not aid in accurately characterizing the performance of the reticle or the stepper system as a whole.
In addition to stepper systems, scanning step and repeat systems 150 (often called "step and scan systems") have become popular, as illustrated in a simplified, exemplary prior art FIG. 3. Step and scan systems differ from the scanning system of FIG. 2c because instead of the entire wafer being scanned with a mask, each die on the wafer is scanned with a reticle and the system then steps across the wafer and scans each die across the wafer. In prior art FIG. 3, a reticle and substrate subsystem 152 are positionally fixed with respect to one another, and the reticle/substrate subsystem 152 are laterally scanned in the X-direction across a slit 154 in the imaging system 156 by laterally moving the subsystem 152. Consequently, in the step and scan system 150, the entire reticle field is not printed at one time, but rather the portion of the reticle field underneath the slit 154 is printed and the slit 154 is scanned across the reticle field by the lateral movement of the reticle/substrate subsystem 152. In the above manner, the exposed portion of the reticle field is always within the same fixed portion of the image field in order to maintain the printing within the "sweet spot" of the imaging optics.
Although step and scan systems constitute an improvement in pattern transfer quality, the scanning process can contribute to pattern transfer errors (and thus variations in the critical dimension along the scan direction (e.g., the X-direction)) if the subsystem 152 positioning is not proper for each scan. For example, if the repeated scans of the subsystem 152 (containing the wafer and reticle) past the slit 154 are crooked or misaligned or the position of the subsystem 152 is rotated slightly from its desired position in the "X-Y" plane, errors may be generated in the transferring of the reticle pattern to the wafer. Furthermore, scanning instability can lead to errors in pattern transfer. In addition, the scanning errors are independent of, and in addition to, the critical dimension variations discussed above with respect to the reticle and the imaging system. Again, as designers wish to make further developments in both the printing systems and the reticles, it is desirable to identify separately the critical dimension contributions associated with the printer (non-reticle contributions) from the critical dimension contributions associated with the reticle in order to accurately characterize the printing system.
Therefore there is a need in the art for a method of separating reticle critical dimension components from printing system critical dimension components in lithographic printing systems.